1. Field of the Invention
The present invention relates to an internal combustion engine, and more particularly, to an internal combustion engine of which a cylinder axis is slanted.
2. Background Art
Variety of internal combustion engines whose cylinders axes are slanted with respect to a vertical axis have been proposed to reduce the overall height of the engines.
For example, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-362270 discloses a slanted SOHC (Single Over-Head Cam) internal combustion engine of which a camshaft is provided above the cylinder axis so that the intake and exhaust valve train is disposed at a low position.
Further, Japanese Patent Application Laid-open No. Toku-Kai-Hei 10-280932 proposes a slanted OHC (Over-Head Cam) internal combustion engine, in which intake and exhaust valves are disposed below the cylinder axis, a rocker arm is disposed below the intake and exhaust valves, and a push rod is accommodated in a push rod chamber formed along the lower surface of a cylinder. The push rod is connected at the lower end thereof with a tappet driven by a camshaft disposed in a crankcase.
Generally, a valve train chamber for accommodating a rocker arm is formed above a cylinder head. Accordingly, as shown in the disclosure No. Toku-Kai-Hei 4-362270, in case of an engine whose camshaft is disposed above the cylinder axis, a large diameter of a camshaft sprocket projected above makes it difficult to reduce the overall height of the engine.
On the other hand, as disclosed in Toku-Kai-Hei 10-280932, in case of an engine whose rocker arm is disposed below the intake and exhaust valves, since the rocker arm is supported by a rocker shaft provided in the cylinder head, the valve train chamber encloses at least the lower half of the cylinder head, as a result it is impossible to dispose a spark plug at an optimal position for combustion. Particularly, in case of an air-cooled engine, the positional selection of the spark plug is a big problem for its cooling characteristic.